The use of sub-critical/supercritical water decomposition reactions has been known as a method for recovering useful substances from wastes such as food wastes and wood wastes. For example, there has been a method in which cellulose, starch, and the like contained in a large amount in biomass are subjected to decomposition reactions with sub-critical/supercritical water to decompose them into monosaccharides or oligosaccharides and the decomposed products are then recovered. The resultant monosaccharides and oligosaccharides are further subjected to alcoholic fermentation or methane fermentation to convert them into ethanol and methane gas, which are easier to utilize (cf. Patent Reference 1). In this method, waste such as wood waste is grinded into fine particles in advance, and thereafter the resultant is mixed with water to form a slurry, followed by decomposition reaction with sub-critical water/supercritical water.
In this method, however, the decomposition reaction is difficult to control. If the reaction takes place excessively, the waste will be decomposed into carbon dioxide and water and useful substances cannot be recovered. Conversely, if decomposition is insufficient, a large amount of residue will remain, degrading the process efficiency. It is believed that the reason why the decomposition reaction is difficult to control with this method is that the hydrolysis reaction with sub-critical water proceeds on the surface of the solid components of the waste. For example, in cellulose decomposition with sub-critical water or supercritical water, the decomposition reaction rates of the decomposition products, monosaccharides and oligosaccharides, are faster than the decomposition reaction rate of cellulose itself. For this reason, the monosaccharides and oligosaccharides are decomposed into organic acids, carbon dioxide, or water; or they are carbonized by polymerizing each other.
To resolve this problem, the following method has been proposed (cf. Patent Reference 2). First, cellulose powder is put into a tube furnished with sintered filters with a pore diameter of 5 μm attached to both ends thereof, and while water in a sub-critical condition is being passed continuously therethrough, oligosaccharide is produced on the cellulose powder surface by the decomposition is extracted with the use of sub-critical water. Next, the sub-critical water is taken out of the reactor while being quenched to thereby prevent side reactions, such as decomposition of the oligosaccharide. This method employs, however, a batch-type reaction and is therefore not suitable for large-scale operations. Moreover, a problem is that the pores in the sintered filters tend to be plugged by various reaction products.
Another method that has been proposed involves decomposing and solubilizing electronic appliance scraps, waste plastics, and the like in a fluidized bed with a special shape, using supercritical water, to separate inorganic solid matter, and thereafter, completely decomposing the resultant into gases and mineral salts with a high-pressure reactor (cf. Patent Reference 3). The apparatus used for this method comprises a columnar fluidized bed reactor having a vertical partition wall, and while supercritical water is being pressure-fed from a lower part of the reactor, slurry in which a grinded waste material is dispersed in supercritical water is pressure-fed into the reactor from the top portion thereof. As a result, the waste containing organic components goes down in the reactor, then passes under the vertical division wall, and further flows into the opposite side of the reactor. At this time, the organic components dissolve in the supercritical water and therefore flow out from the top portion of the reactor. To the organic components dissolving in the supercritical water, an oxidizing agent is further added in the high-pressure reactor so as to be completely decomposed into gases, such as carbon dioxide, and mineral salts. The inorganic powder, such as ceramic, that is contained in the waste is discharged from the top portion of the fluidized bed reactor. In this method, organic matter is decomposed and solubilized by supercritical water while waste material is traveling through a predetermined flow path in the fluidized bed reactor. This method uses, as the reaction and extraction solvent, water in a supercritical state, which has a low density, and is therefore suitable for completely decomposing ceramics, waste plastics, and the like, which have a high density, into gases and mineral salts, or isolating inorganic solid substances.
Nevertheless, because organic wastes that contain solid matter with a high water content has a density close to that of supercritical water, this method is problematic when decomposition reactions need to be controlled or when a specific decomposition product needs to be obtained at a high yield rate. Moreover, this method requires a complicated apparatus configuration since supercritical water and organic waste material are pressure-fed through different charging ports to the fluidized bed; for the pressure-feeding, at least two high-pressure pressure-feeding apparatuses (compressing means), which are expensive, are necessary. Furthermore, the material for the reactor needs to be resistant to corrosiveness of supercritical water, and therefore, it is necessary to use expensive materials such as Hastelloy and Inconel as the material for the reactor. This increases the equipment costs.
Another method has been proposed in which mixed slurry of sub-critical water and organic waste is introduced into a pressure-resistant reactor by spraying from the top of the reactor, to thereby treating organic wastes by decomposition (Patent Reference 4). Nevertheless, even with this method, it is difficult to control the decomposition of organic waste material.
[Patent Reference 1]    JP 2001-262162A
[Patent Reference 2]    JP H10-327900A
[Patent Reference 3]    JP 2002-210348A
[Patent Reference 4]    JP 2001-246239A